Concept of a xenobiotic and the general detoxication strategy
Xenobiotic (literally meaning "foreign substance") is any compound that enters the body with food, through the skin, or via the lungs and is not used for energy or structural purposes. Most xenobiotics are hydrophobic and potentially toxic, so the body must convert them into a water-soluble form for subsequent excretion in urine or bile.
Biotransformation is carried out by microsomal oxidation enzymes localized in the endoplasmic reticulum of hepatocytes and proceeds in two consecutive phases: the modification phase (introduction of a polar group) and the conjugation phase (attachment of a hydrophilic partner).
Phase One: Microsomal Oxidation
The first phase is mandatory for hydrophobic substrates and most commonly involves hydroxylation. A key role is played by the monooxygenase system, which includes cytochrome P450 and cytochrome P450 reductase.
Reaction mechanism: cytochrome P450 reductase oxidizes NADPH to NADP⁺, transferring two electrons to an oxygen molecule. One oxygen atom is reduced to water, while the second is incorporated into the substrate to form a hydroxyl group. Overall equation:
- RH + O₂ + NADPH + H⁺ → R–OH + H₂O + NADP⁺
Cytochromes P450 possess a relatively broad substrate specificity: a single isoform is capable of oxidizing both a drug and a poison. With systematic intake of toxic substances, induction of cytochromes occurs, which accelerates oxidation and underlies the development of tolerance.
Phase Two: Conjugation Reactions
In the second phase, hydroxylated metabolites combine with polar partners involving transferases. Conjugating agents can include:
- glucuronic acid (UDP-glucuronosyltransferase)
- sulfate in its active form PAPS (3'-phosphoadenosine-5'-phosphosulfate), transferred by sulfotransferases
- glycine, acetate, methyl groups
- glutathione (γ-Glu-Cys-Gly) — transferred by glutathione S-transferases
Conjugation products become sharply hydrophilic and are easily excreted. Glutathione S-transferases are represented by numerous isoforms and participate in the detoxication of steroid hormone metabolites, prostaglandins, bilirubin, bile acids, and lipid peroxidation products. In erythrocytes, glutathione is additionally used to reduce organic peroxides to alcohols. Phase two enzymes are also inducible during chronic intake of alcohol, drugs, and poisons.
Ethanol Detoxication
Normally, ethanol is oxidized in the hepatocyte cytosol via an NAD⁺-dependent pathway:
- Alcohol dehydrogenase: ethanol + NAD⁺ → acetaldehyde + NADH
- Acetaldehyde dehydrogenase: acetaldehyde + NAD⁺ → acetic acid + NADH
Acetic acid is subsequently activated to acetyl-CoA and enters the Krebs cycle or lipogenesis.
During regular ethanol consumption, the microsomal ethanol-oxidizing system (MEOS) is activated, with the cytochrome P450 2E1 (CYP2E1) isoform playing a key role. This enzyme oxidizes ethanol directly to acetaldehyde, consuming NADPH and oxygen. CYP2E1 simultaneously metabolizes a number of drugs, including hypnotics. The induction of MEOS explains why individuals who regularly consume alcohol require higher doses of drugs to achieve a therapeutic effect — this leads to the development of cross-tolerance.
Chemical Carcinogenesis: Nitrates and Nitrosamines
Carcinogenesis is damage to the structure of DNA (as well as RNA and proteins) leading to malignant cell transformation. Several groups of chemical carcinogens are distinguished.
Nitrates enter the body via water, vegetables, and preserved meat products. In the body, they are sequentially reduced (gaining 2 electrons at each step) through nitrites, NO, and hydroxylamine. These metabolites:
- oxidize the iron in heme-containing proteins (hemoglobin with the formation of methemoglobin, electron transport chain cytochromes, cytochromes P450)
- decrease the activity of the antioxidant system, promoting the accumulation of reactive oxygen species and lipid peroxidation
- participate in the pathogenesis of atherosclerosis
Nitrosamines, formed from nitrites and secondary amines, cause alkylation of DNA nitrogenous bases. Deamination of cytosine converts it into uracil, which pairs with adenine during replication, leading to a substitution with thymine in the next cycle — fixing a point mutation.
Aflatoxin B1 and Aromatic Carcinogens
Aflatoxin B1 is produced by the mold Aspergillus flavus during improper storage of grains, nuts, and peanuts. In the liver, aflatoxin undergoes microsomal oxidation by cytochrome P450 to form a reactive epoxide. The epoxide covalently binds to DNA guanine, causing mutations, and is one of the most potent natural carcinogens, leading to hepatocellular carcinoma.
The paradox of the detoxication system lies in the fact that phase one enzymes can not only detoxify but also activate procarcinogens, converting them into reactive electrophilic metabolites. The protective role of phase two (especially glutathione S-transferase) consists of inactivating these intermediates before they bind to DNA.
Key points
- Xenobiotic detoxication proceeds in two phases: modification (hydroxylation) and conjugation.
- Phase one utilizes the monooxygenase system: cytochrome P450 + P450 reductase + NADPH + O₂.
- In phase two, transferases attach glucuronate, sulfate (PAPS), glutathione, glycine, or acetate.
- Enzymes of both phases are inducible — this is the molecular basis of tolerance to alcohol and medications.
- Ethanol is oxidized by alcohol and acetaldehyde dehydrogenases; chronic intake recruits MEOS involving CYP2E1.
- Nitrates damage heme-containing proteins and activate lipid peroxidation; nitrosamines alkylate DNA.
- Aflatoxin B1, following activation by cytochrome P450, is converted into an epoxide — a potent liver carcinogen.
- Glutathione S-transferases provide universal defense against electrophilic metabolites and lipid peroxidation products.